From the viewpoint of global environmental preservation, the automobile industry recently remains confronted with an important challenge of enhancing the fuel efficiency of automobiles to reduce carbon dioxide CO2 emissions. Saving weight in automobile bodies is an effective approach to improve automobile fuel efficiency. The weight reduction of car bodies needs to be accompanied by maintenance of the strength of automobile bodies. Lightweight car bodies may be realized without decreasing their strength by increasing the strength of steel sheets as automobile part materials so that the thickness of the materials can be reduced. From this viewpoint, there has recently been a very strong demand that such part materials have higher strength, resulting in an increasing use of high-strength thin steel sheets as such part materials.
However, application of high-strength steel sheets to such parts is frequently interfered with by the presence of variations in strength and workability in individual high-strength steel sheets, namely, variations in mechanical properties in individual steel sheets (steel strips). Variations in strength induce varied amounts of spring back to destabilize the shape of press-formed parts. Further, variations in strength give rise to variations in stretch flangeability and can cause fractures during press forming.
In general, variations in steel sheet strength are ascribed to variations in temperature history experienced in the rolling direction and in the width direction of the steel sheet during the manufacturing of steel sheets, and further ascribed to variations in steel sheet microstructure produced by differences in rolling conditions.
To address these problems, for example, Japanese Unexamined Patent Application Publication No. 2007-308771 describes a high-strength steel sheet with a tensile strength of not less than 500 MPa which includes not less than 60% of a ferrite phase. That steel sheet is characterized in that when the steel sheet is deformed with a strain of 20% or more, the deformed region contains at least 50% of ferrite crystal grains in which dislocation cell structures arranged in one direction intersect with other such structures in at least two directions. According to the technique described in JP '771, the amount of spring back that occurs after the forming of parts can be stably reduced, namely, parts with excellent shape fixability can be produced. However, the steel sheet according to that technique contains, in addition to ferrite, a hard phase that affects the strength of the steel sheet, and the amount of such a hard phase is caused to significantly fluctuate by differences in manufacturing conditions from place to place in the steel sheet during manufacturing on the industrial scale. This fact problematically causes significant variations in steel sheet strength within the steel sheet (the coil).
Japanese Unexamined Patent Application Publication No. 2004-250743 describes a high-workability high-strength hot rolled steel sheet with excellent shape fixability and small anisotropy. The high-strength hot rolled steel sheet obtained according to the technique of JP '743 has a microstructure containing a ferrite or bainite phase with the largest volume fraction or further contains 1 to 25% of martensite and retained austenite and in which a group of specific crystal orientations of the sheet surface at ½ sheet thickness has an average ratio of X-ray intensity to a random sample of not less than 2.5, specific three crystal orientations at ½ sheet thickness have an average ratio of X-ray intensity to a random sample of not more than 3.5, at least one of the r value in the rolling direction and the r value in a direction perpendicular to the rolling direction is not more than 0.7, and the anisotropy in uniform elongation ΔuEl is not more than 4% and not more than the anisotropy in local elongation ΔLEl. Those configurations allegedly realize thin steel sheets having good press formability with a small amount of spring back, namely, excellent shape fixability and also with small anisotropy. However, the technique described in JP '743 has problems in that the texture of the steel sheet cannot be obtained stably in the longitudinal direction and the width direction of the coil and further that the positive formation of martensite and retained austenite in the steel sheet microstructure results in a marked decrease in the stability of strength to make it very difficult to obtain stable shape fixability.
Japanese Unexamined Patent Application Publication No. 2003-321734 describes a high-formability high-tensile strength hot rolled steel sheet having excellent uniformity in quality. According to the technique described in JP '734, a steel containing C: not more than 0.1%, Ti: 0.02 to 0.2% and one or both of Mo and W to satisfy a specific relation of the Ti, Mo and W contents is hot rolled, coiled into a coil and heat treated to produce a steel sheet that has a microstructure substantially composed of ferrite in which a carbide precipitate containing titanium and one or both of molybdenum and tungsten is dispersed. That steel sheet is described as having an excellent uniformity in quality such that the difference in yield stress between a widthwise central portion and a widthwise end portion of the steel sheet is not more than 39 MPa. Although the technique described in JP '734 can reduce quality variations in the width direction to a certain extent, the segregation of manganese causes tensile strength to vary from place to place in the longitudinal direction of the steel sheet (the coil). Thus, the uniformity in quality remains to be improved.
Japanese Unexamined Patent Application Publication No. 2003-321735 describes a high-formability high-tensile strength steel sheet with excellent stability in strength. According to the technique described in JP '735, the steel sheet has a chemical composition including C: 0.03 to 0.15%, Mn: not less than 0.2%, N: not more than 0.01%, Ti: 0.05 to 0.35% and one or both of Mo: not more than 0.6% and W: not more than 1.5%, the contents of molybdenum and tungsten, when contained solely, being Mo: not less than 0.1% and W: not less than 0.2%, the Ex. C content (the content of carbon not bonded to titanium, molybdenum or tungsten) being not more than 0.015%, the Mn content satisfying a specific relationship with the Ex. C content. Further, the steel sheet has a microstructure substantially composed of ferrite in which a precipitate with a size of less than 10 nm containing titanium and one or both of molybdenum and tungsten is dispersed. According to that disclosure, the high-tensile strength steel sheet having the above configurations exhibits a tensile strength of not less than 550 MPa and achieves excellent strength stability. When, however, the Mn content is 1% or more, the steel sheet decreases strength stability due to the segregation of manganese and cannot maintain the stability of strength in the width direction.
Japanese Unexamined Patent Application Publication No. 2002-363693 describes a high-stretch flangeability steel sheet with excellent shape fixability. According to the technique described in JP '693, the steel sheet is configured such that a ferrite or bainite phase has the largest area fraction, the occupancy proportion of iron carbide in grain boundaries is not more than 0.1, the maximum particle size of the iron carbide is not more than 1 μm, the steel sheet has a texture in which crystals with specific orientations are aligned in parallel with at least the sheet plane at the center of the sheet thickness, and the r value is controlled in a specific range. Those configurations are described as reducing the amount of spring back and improving shape fixability. However, it is difficult with the technique of JP '693 to stably ensure the specific texture in the longitudinal direction and in the width direction of the coil. Thus, a difficulty remains in obtaining steel sheets with stable strength.
Japanese Unexamined Patent Application Publication No. 2011-26690 describes a low-alloy high-strength hot rolled steel sheet containing, by mass %, C: 0.02 to 0.08%, Si: 0.01 to 1.5%, Mn: 0.1 to 1.5% and Ti: 0.03 to 0.06%, the ratio of the Ti content to the C content being controlled to Ti/C: 0.375 to 1.6, and in which the size and the average number density of TiC are 0.8 to 3 nm and not less than 1×1017 particles/cm3, the steel sheet having a tensile strength of 540 to 650 MPa. According to the technique described in JP '690, TiC is finely dispersed by performing coiling at a temperature of not more than 600° C., thereby ensuring a high strength of not less than 540 MPa in terms of tensile strength. However, although the size of the precipitate is limited to 0.8 to 3 nm, significant fluctuations are caused in terms of yield strength which is more sensitive to variations in the size of precipitates than tensile strength. Further, as illustrated in EXAMPLES of JP '690, ensuring a tensile strength of not less than 590 MPa requires a coiling temperature of not more than 575° C. and also a Mn content of not less than 1% or a C content of not less than 0.07%. Thus, the disclosed technique has a problem in that strength cannot be obtained stably.
Japanese Unexamined Patent Application Publication No. 2007-247046 describes a high-strength steel sheet with excellent strength-ductility balance. The technique described in JP '046 resides in a hot rolled steel sheet with excellent strength-ductility balance which contains, by mass %, C: 0.01 to 0.2%, Mn: 0.20 to 3% and one, or two or more of Ti: 0.03 to 0.2%, Nb: 0.01 to 0.2%, Mo: 0.01 to 0.2% and V: 0.01 to 0.2%, and is configured such that the steel sheet includes a ferrite single phase microstructure that contains two kinds of crystal grains, namely, hard ferrite crystal grains A and soft ferrite crystal grains B having different number densities of 8 nm or finer precipitate or cluster particles in the crystal grains. That technique simulates and reproduces the working hardening behavior of DP steel by changing the hardnesses of the crystal grains. However, the technique of JP '046 involves a large amount of silicon or aluminum singly or in combination with each other, and describes that the use of such large amounts of silicon and aluminum is essential to achieve the distribution of 8 nm or finer precipitate or cluster particles satisfying the prescribed number densities. According to the technique of JP '046, a Mn content of 0.87% or above is required to ensure strength as illustrated in its EXAMPLES. Further, the technique described in JP '046 has a problem in that controlling of the cluster distributions in the respective crystal grains is contributory to the development of variations in strength among the crystal grains, and consequently the coil fails to attain stable quality.
JP '771, JP '743, JP '734, JP '735, JP '693, JP '690 and JP '046 assert that higher strength and improvements in workability and shape fixability are generally expected according to the techniques described therein. However, individual steel sheets (coils) obtained by any of those techniques show significant variations in strength. Because of the instability in strength, parts (components) fabricated from a single steel sheet (coil) have different dimensional accuracies. Thus, it has been difficult to manufacture parts with stable dimensional accuracy.
It could therefore be helpful to provide high-strength hot rolled steel sheets with excellent stretch flangeability which have small variations in mechanical properties in individual coils and thus allow parts to be fabricated therefrom with stable dimensional accuracy, and also to provide methods of manufacturing such steel sheets. The term “high-strength hot rolled steel sheets” refers to hot rolled steel sheets with high strength which have a yield strength YS of not less than 530 MPa and preferably have a tensile strength TS of not less than 590 MPa. The phrase “having small variations in mechanical properties in individual coils” means that the difference in yield strength YS, ΔYS, between a widthwise central portion and a widthwise end portion of a steel strip in the form of a coil is not more than 20 MPa as will be described later in the EXAMPLES.